The concepts herein relate to gaseous fuel combustion for internal combustion engines.
There is a push to utilize natural gas as an engine fuel due to its low cost. Relative to diesel fuel, for instance, natural gas is a lower cost fuel per energy. Natural gas is typically ignited with a spark plug. However, the ignition energy from the spark plug does not always effectively ignite the natural gas at high compression ratios, particularly at lean operating conditions. To remedy this, some systems forgo a spark plug and use diesel fuel as a pilot fuel. In other words, these systems inject a small amount of diesel fuel as a pilot fuel early in the compression cycle that auto-ignites from the compression. Natural gas is then by injected and ignited by the combusting pilot fuel. However, a natural gas system using diesel as a pilot fuel requires two fuel systems and associated piping, storage, injectors, etc., which can increase cost, size, complexity and makes retrofitting difficult.
Compression ignition (e.g. Diesel) engines are known to be the standard for efficiency—due to high compression ratio, induction and compression of air rather than fuel and air, no throttle and high brake mean effective cylinder pressure (i.e., “BMEP”). However, the fuel in a typical diesel engine burns in a diffusion flame—having a fuel rich core injected into air. The flame front is at the stoichiometric interface between fuel and air and the rate of combustion is controlled by the rate that oxygen diffuses into the reaction and burned products diffuse out of the reaction zone. This non-homogeneous combustion leads to (a) high emissions of NOx and soot and (b) slower burning and thus less efficient overall combustion phasing, since significant fraction of the combustion occurs as the piston is expanding, so that each gram of fuel that burns after the crank top dead center will enjoy a lower and lower expansion ratio and thus lower work extraction.
Alternatively, spark ignition combustion of gaseous fuels within an Otto Cycle (near constant volume combustion) has highly efficient combustion and good combustion phasing for efficiency, however due to the auto-ignition process inherent in premixed air-fuel mixtures—to control detonation and knock the compression ratio must be lower; a throttle is used to control load; and the fresh charge of air plus fuel is the compression working fluid. However, when operated at controlled stoichiometric air-fuel ratio (i.e., AFR), a low-cost three-way catalysis is very effective in reducing emissions to regulated levels without much complexity.